Participants asked to hold a pen between the teeth, forcing a smile, rated cartoons as funnier than those holding it in their lips. What does the facial feedback hypothesis claim about this?

D. That expression feeds back into experience, not only displays it

The claim is that the arrow runs both ways, expression is an input as well as an output. The pen exists to prevent demand characteristics, since nobody is told to smile. Worth noting the effect has replicated inconsistently, which is why it is now stated more cautiously than in the textbooks.